Vocoder filter system



United States Patent inventor Donald Anthony Acott Roworth ReferencesCited London, England UNITED STATES PATENTS Appl. No. 649,518 2,819,3411/1958 Barney l79/ltASl Filed June 28,1967 3,330,910 7/1967 Flanagan179(AS)X Paemed Dec-29,1970 3,423,530 l/1969 Coulter l79/|(AS) AssigneeInternational Standard Electric Corporation Primary Examiner-Kathleen H.Clafiy New York, Assistant Exmcinerjfilarles WhJirazuch P M PAnorneysome emsen, r., ayson orris. ercy priority 3:333:22 of DelawareP. Lantzy, .1. Warren Whitesel, Phillip A. Weiss and Delbert GreatBritain Warner Nos. 29684/66,29685/66 and 29686/66 ABSTRACT: A vocoderor voice synthesizer includes a plurality of filters which are used toanalyze human speech signals so that they may be synthesized. Theoriginal voice signal is se arated b a plurality of hi h Q-filters. Avariable resistor is VOCOPER FILTEI} SYTEM co t ipled acrbss thesefilters t5 selectively increase or decrease l3 Chums n D'awmg thequality factor or Q of the filters. Time controlled clamping U.S. Cl.179/1, means are provided for alternately sampling speech via high179/1555 Q-filters and then discharging the filter elements by loweringInt. Cl G101 1/00 the O. This way, the energy stored in the filtersduring one Field ofSearch 179/1AS; speech mpl does n degrade theresponse to the next speech sample.

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PATENTEU UEC29 I978 SHEET 1 OF 7 N Wm 2% Ni tn fi fi m: m M w w w w i wa E E m w w w E w T @ii Ws N N xiv a m m m H/ -%/s e Input PATENTEDDEE29 I970 3551.588

SHEET 2 OF 7 [70/0 in Mal/1y Pulse #520? ('oml' o/ Voltage I A L VO/tSPATENTEU 0Ec29 I970 3551.588

' SHEET 5 OF 7 PATENTEU UECZSIBYG 3,551,588

SHEET 7 OF 7 VOCODER FILTER SYSTEM The present invention relates tocircuitry and methods for periodically sharply varying the qualityfactor of resonant circuits so as to allow the stored energy of thecircuit to be disrsipated and has particular though not exclusiveapplication in a speech synthesizer or in the synthesizer stage of avocoder.

A vocoder is a speech transmission system comprising an analyzeriwhichproduces control signals derived from a speech inputya' transmissionline for the said control signals, and a synthesiz 'eiifi i Thebandwidthprequired to, transmit the control signals is less thanthat"required to transmit the speech in analogue form. The controlsignals cari -be multiplexed, and are not subject to the same kinds ofdistortion as are analogue speech signals. in a voice excited vocoder,for example, a base band covering a frequency range such as 80600c.p.s.is transmitted in analogue form and is added at the synthesizer to theoutput of the parallel frequency channels which cover the remaining partof the speech bandwidth. The base band is also I subjected at thesynthesizer to nonlinear distortion to provide the excitation signal forthe stimulation of the frequency channels.

' Other types of vocoder include, for example, the correlation vocoderand the formant tracking vocoder. v

The synthesizer of many types of vocoder consists of a number offrequency channels in parallel, each channel containing a modulator anda band-pass filter. A signal whose pitch is that of the original speechand which contains sufficient harmonic content to cover the frequencyrange of interestis used to excite the channels. Commonly the abovesignal consists of a train of narrow pulses which in the case of a fullchannel vocoder are spaced at pitch rate and in the case of avoice-excited vocoder are spaced at a rate which is related to pitchrate but not necessarily equal to it. If the channel filters are sharplytuned (as is generally the case), they will ring after theapplication ofeach pulse for a time greaterthan the time between pulses. There willthus be energy remaining in the filters when the next pulse is applied,and depending on the relative phase of the damped oscillation it willinterfere additively or othenvise with the response of the resonantcircuit to the succeeding excitation signal.

The present invention is suitable for use in the synthesizer of achannel vocoder which is excited by a series of pulses and in thechannel portion of the synthesizer of a voice-excited vocoder.

-ln vocoders which employ single tuned filters for the frequencychannels in the synthesizer stage and wherein sam- ,ple excitationpulses are applied to the filters once in every propose therefore toemploy a technique which we refer to as clamping" which consists ofallowing the filter to be of as high a during the major portion of eachpitch period as is required and considerably reducing the Q at the endof each pitch period for a short time immediately before the applicationof the next excitation pulse, thus ensuring that there is lit tle orno-energy remaining in the filter when it is again excited.

The invention provides a method of dissipating the energy stored in aresonant circuit in a vocoder synthesizer without giving rise toundesirable transients in the output.

The duration of the clamping signal is as short as possible whilebeingsufficiently long to allow substantially all of the stored energyto be dissipated and the signal is timed so as to end at the applicationof the succeeding excitation pulse.

Various preferredernbodiments of the inventioninclude a number ofvariable bandwidth filters whose bandwidths are variable independentlyof one another or a number of filters the bandwidths of some or all ofwhich are variable in concert.

The resonant frequency f in cycles per second of a parallel LC circuitis given approximately by 2 v26 Awhere L and C are the values in M.K.S.units of the inductance and the capacitance respectively.

The range of frequencies around the resonant frequency over which theresponse of the resonant circuit does not fall more than 3 decibelsbelow the value of the output at resonance is called the bandwidthand isdenoted by 8]".

The quality factor Q of the circuit is defi'ne'd'bv the formula Q=i I. i:7 I I 6f 1(2) where f is the resonant frequency. Q is thus 'a measureof resolution. 0 may be shown to satisfy the relation if C has beenchosen to give the required f.

Thus the bandwidth is proportional to the overall resistance of thecircuit. if the resistance is arranged to be variable in response to acontrol signal, we obtain a filter whose bandwidth is externallycontrollable. i

For a mathematical analysis of resonant circuits, reference may be made,for example, to 'J. Millman and H. Taubs Pulse, Digital and SwitchingWaveforms; devices and 'currents for their generation and processing(McGraw-Hill 1965). Obviously the complete mathematical analysis of aseries resonant circuit will differ from that of a parallelresonantcii'cuit. However the techniques of clamping andof smoothlyvarying the bandwidth of a filter which are provided by thepresentinvention have application both in series and in parallel tunedcircuits.

In one version of the preferred embodiment of the invention, thebandwidth of a single tuned LC filter isaltered by changing theeffective resistance in the circuit.

A resonant circuit with positive damping is one in which the outputdecays with time, e.g. the output might be of the form exp (-kt). sin (Ic) where exp denotes the exponential function, e is a constant, k is apositive constant called the .decay factor, and t is time measured fromthe moment of application' of the excitation pulse.

The circuit is said to be critically damped when the rate of energydecay is a maximum. In this case the energy is dissipated in the minimumpossible time.

Any real resonant circuit has inherent damping due to resistive lossesin the circuit elements, particularly in the inductance. Generally it isrequired to minimize this damping in order to realize the maximumpossible selectivity, but if an additional resistance is switched inparallel with a branch of a resonant circuit, for example, the resultwill be critical damping of the circuit if the additional resistance hasbeen selected appropriately.

According to the invention there is provided a circuit arrangement whichincludes a bank of resonant circuits resonating each at a differentfrequency and having quality factor control means whereby the bandwidthof one ofthe resonant circuits may be varied.

The invention resides in the method employed and in circuitry'orapparatus which includes a synthesizer of the type described, and in aresonant circuit wherein critical damping is introduced by the methoddescribed.

' In one embodiment of the invention, the critical damping resistance ofthe resonant circuit is introduced by being switched in electronically.

In a particular embodiment of the above paragraph a symmetricaltransistor is in series with a resistor, a pulse of the appropriateduration being applied at the appropriate moment to the base of thetransistor so as to switch into the resonant circuit the resistance ofthe transistor together with the resistance of the resistor.

The polarities of the power supply to the transistor, of the resonantcircuit, of the excitation pulses and of the pulse applied to the baseof the transistor are not necessarily independent of one another.

In a preferred embodiment of the invention, the bandwidths of theresonant circuits of the speech synthesizer are variable independentlyof one another, so that, for example, subjective tests can be made as towhat settings give rise to most acceptable speech. enabling researchinto acceptableness and other characteristics of speech to be carriedout.

The tenn "speech synthesizer includes, for example, the synthesizerstage of a vocoder, a reading machine for the blind, and a parametricartificial talker.

The above-mentioned and other features of the invention and the mannerof attaining them will become more apparent and the invention itselfwill be best understood by reference to the following description of anembodiment of the invention taken in conjunction with the accompanyingdrawings, in which:

FIG. I is a block diagram of a channel vocoder;

FIG. 2 is a circuit diagram of one form of filter for a vocodersynthesizer channel;

FIG. 3 is a circuit diagram which is equivalent to a part of FIG. 2;

FIG. 4 is a circuit diagram which is equivalent to a different part ofFIG. 2;

FIG. 5 is a circuit diagram of a second form of filter for a vocodersynthesizer channel with clamping facilities;

FIG. 6 is a circuit diagram of a second form of filter for a vocodersynthesizer channel with bandwidth control;

FIG. 7 is a graph illustrating the variation in bandwidth in thecircuits of FIGS. 2 and 4 for a range of applied quality factor controlvoltages;

FIG. 8 shows oscilloscope traces of output waveforms obtained forvarious control voltages in the circuit of FIGS. 2 and FIG. 9 is anoscilloscope trace showing a clamping signal and the output obtainedwhen the clamping signal is applied;

FIG. 10 is an oscilloscope trace of the output obtained with zeroclamping signal; and

FIG. I1 is a block diagram of a parametric speech synthesizer.

The definitions of some terms used in the following description will nowbe introduced.

A symmetrical transistor is one which is constructed in such a way thatits characteristics remain substantially unchanged when the emitter andcollector connections are interchanged.

A transistor is said to be saturated when the difference in potentialbetween collector and emitter is less than that between base andemitter. When this occurs, the transistor undergoes bottoming, that isto say the output is determined by the characteristics of the transistorand not by the applied signal.

A symmetrical transistor operating in the saturated mode has aneffective emitter-collector resistance R given by where I,, is the basecurrent.

B the common emitter current gain, and

A is typically 40 per volt at I 6 C.

For a typical transistor of the kind utilized as 05 or 06 in thecircuits of FIGS. 2, 3, and 4, or as 050 in FIG. 5 or as 060 in FIG 6. Ris about l0 ohms when I,, is 100 p. A.

FIG. 1 illustrates a so-called channel vocoder having an analyzer stagefrom which signals are derived and carried by a number of transmissionpaths, one of which, 10, carries the fundamental frequency informationof the speech, while the remainder convey the spectral information.These signals control the synthesizer stage which reconstructs theoriginal speech. In more detail, a speech signal is picked up by amicrophone II and amplified by an amplifier 12. The output of theamplifier I2 is fed to a pitch detector 16 which detects whether thefundamental voice frequency is random or periodic. From 16 the pitchinformation goes to a frequencyto-voltage converter 17. The output of 17is fed through a lowpass filter 18 and carried by the pitch channel 10(which thus conveys the fundamental frequency or pitch information ofthe speech, if any) to a switch 19 and a pitch generator 20. If thepitch information is zero, a hiss generator 21 is switched on. If thepitch information exceeds a threshold value, "voicing" in the -300c.p.s. range is provided.

The output of the amplifier 12 goes also to a pre-emphasis equalizer 13and thence to a bank of analyzer spectrum filters ASF whose outputs arerectified by the bank of rectifiers RECT and then applied to the bank oflow-pass filters LI- -L9. The signals carried by channels 1-9 thusconvey the spectral information of the speech. The outputs of channels1- 9 are applied to the modulators MlM9 where they modulate the pitch orhis signal from 20 or 21 respectively. The modulated outputs of Ml-M9pass through a bank of synthesizer spectrum filters SSF to be combinedat the deemphasis equalizer l4, whence the reconstituted speech signalgoes to an amplifier 15, being output in audible form at the headphones22.

The technique of switching in of critical damping can be utilized insome or all of the synthesizer spectrum filters SSF.

The bandwidths of the analyzer spectrum filters ASF can be variedindependently of one another or in concert.

If the synthesizer spectrum filters are of the type shown in FIG. 2,then variation of bandwidth can be achieved in filters which can also becritically damped between successive excitation pulses.

FIG. 2 (and FIGS. 3 and 4, which are different portions of FIG. 2,corresponding respectively to the clamping facility and to the controlfor smoothly varying the bandwidth) illustrates one spectral filter andsome associated circuitry from the filter bank SSF of the synthesizerstage of the vocoder of FIG. 1, the filter consisting of a resonantcircuit. A series of excitation pulses spaced at voice fundamental rateare generated (by 20 of FIG. 1, from information provided by l6, l7 and18) and applied to the synthesizer where they excite the tuned circuitswhose outputs are combined to give an analogue response which isconverted to audio form by an electromechanical transducer.

The parallel resonant circuit consists of an inductor LI and a capacitorC1 in the collector circuit of Q4. Excitation pulses are applied to thebase of Q4. The signal output is taken to a high impedance (Q7 of FIG.2, not shown in FIG. 3) to avoid loading the resonant circuit. Clampingis accomplished by placing in parallel with L1 and C1 a resistor R1which is in series with a symmetrical transistor Q5. O5 is normallybiassed to be nonconducting, but the application of a negative pulse ofat least 10 volts amplitude turns it hard on and places the lowresistance R1 across the resonant circuit thus dissipating the energystored therein. The clamping pulse is timed to be just long enough toallow all the energy to be dissipated and to end just before theapplication of the next excitation pulse.

The capacitor C1 is taken to earth through a variable resistance P and asymmetrical transistor Q6.

The variable resistance enables the minimum bandwidth to be preset. Thetransistor is used as a variable resistance, and so must always be keptsaturated. This means that only small signals can be handled and so thegain of the stage is kept low by making the emitter ,resistance of Q4large, say kiloohms.

A voltage applied for quality control delivers a current through thepotential divider formed by R2 and R3 to the base of the transistor 06.For an applied negative voltage in the range 10 volts to 1.5 volts thebandwidth is in the range 60 c.p.s. to 500 c.p.s.

The symmetrical PNP transistor Q6 is used as a variable resistance bykeeping it saturated, in which case the collector current is independentof the base current.

Then the bandwidth '6 f for a given frequency f is given by equation (4)where R is the total resistance of the circuit, and so a; R M

21L (6) (since R R, R where R, is the resistance of the inductor). Bi.e.

where A and B are constants. Thus a parabolic relationship holds between8 f and I, over a certain range.

(The ratio R3/R2 is approximately 10/ l in order to obtain in aconvenient way the range of variation in transistor base current fromO6.)

FIGS. 5 and 6 illustrate as an alternative a series resonant form ofresonant circuit for the synthesizer stage of a vocoder and havingrespectively clamping facilities and a control for smoothly varying thefilter bandwidth. R10, R20, R30, L10, Q50 and 060 correspond to R1, R2,R3, L1, 05 and Q6 of FIGS. 2-4. The effective capacitance C10 of thecircuit of FIGS. 5 and 6 is given by Application of a clamping pulseinput to the base of 050 in FIG. 5 results in critical damping beingintroduced across the resonant circuit for the duration of the pulse.

The duration of the pulse is chosen to be such that all the energystored in the circuit is dissipated in as short a time as possible inthe interval between the application of successive excitation pulses andthe timing is such that the clamping pulse ends before the applicationof the next excitation pulse.

In FIG. 6 the inductance L10 is seen to be provided by the primary.winding of a transformer. Variation in bandwidth is achieved by applyinga negative control voltage through the potential divider formed by theresistance R20 and R30 to the base of a saturated symmetrical transistorQ60.

In FIGS. 2-6 the symmetrical transistors have been PNP transistors. NPNtransistors can be used throughout, (though symmetrical NPN transistorsare' notv readily commercially available at present), appropriatechanges e.g. in the polarity of the power supplies being made in thecircuits. The excitation pulse of FIGS. 2-6 and the clamping pulse ofFIGS. 2, 3 and 5. are negative going signals. The control voltage ofFIGS. 2, 4 and 6 is a negative voltage. These require to be positive ifthe polarities are reversed. Field effect transistors can be used inplace of symmetrical PNP transistors, with appropriate modifications inthe circuits.

FIG. 7 illustrates the control characteristic obtained by applying aquality control voltage as described with reference to FIGS. 2 and 4.The curve is seen to vary smoothly over the range I c.p.s. to 300 c.p.s.which is commonly of most utility in vocoder application.

FIG. 8 illustrates output waveforms obtained by repetitive stimulationof the tuned circuit for various values of the control voltage(equivalently, for various bandwidths).

Referring again to FIGS. 2,3 and 5, clamping" is achieved by placing thecritical damping resistance in series with a symmetrical transistoracross the inductor. The base of O5 is biased so that normally it isturned off, but the application at the input labeled "clamping pulse" ofa negative pulse (of 10 volts amplitude for the filter of FIGS. 1 and 2)turns it hard on and the oscillations of the tuned circuit die awayrapidly. The time required for all the energy to be dissipated dependson the resonant frequency, being greater for the lower frequencies. Forthe lowest frequency channels, about 400 psec. is required, but as thismay represent about 20 percent of the time between excitation pulsessome compromise will be needed. Normally the clamping" pulse should endat or just after the commencement of the excitation pulse.

The upper trace of FIG. 9 illustrates the response of a single filter ofresonant frequency 1,000 c.p.s. which is excited by a train of pulseswhose repetition rate is constant at 250 p.p.s. The lower trace. whichis to be interpreted as a relatively short negative pulse of amplitudel0 volts and not as a long positive pulse illustrates the form of theclamping signal which is applied to the base of 05 so as to preventenergy from being carried over between one excitation period and thenext.

FIG. 10 shows the response of the same filter to the same pulse trainbut with the clamping signal removed. and shows the reduced response dueto destructive interference between the oscillations of successive pitchperiods.

FIG. 11 illustrates a synthesizer which produces artificial speech. Acontrol unit 30 produces control signals, the control unit itself beinggoverned by, for example, a digital computer with paper tape output, orby a voltage reader which reads frequency and spectral informationpresented as manually prepared patterns of electrically conducting inkon rolls of paper. For the production of vowels, a fundamental frequencysignal F is applied to a pulse generator 31, the output of which isapplied to a voice amplitude circuit 32 whose operation is controlled byan amplitude signal A and whose output is fed through a low-pass filter33 and thence through resonant circuits 34 (F3), 35 (F2) and 36 (F1) toan integrator 37, and thence through a network which includes a variableresistance 38 to an amplifier A3, the output being an audible signal.For the production of voiced consonants and front fricatives, a noisegenerator 39 feeds a first noise amplitude circuit 40 which iscontrolled by the parameter Al. The output of 40 passes through a highpass filter 41 and thence goes to the resonant circuits 34 (F3), 35 (F2)and 36 (F1), passing next to the integrator 37 and the variableresistance 38 and finally to the output amplifier A3. For the productionof other consonants, including, for example, the back fricatives, theoutput .through a variable resistor 44 and is then output asan audiblesignal from the amplifier A3.

The above description of a parametric artificial talker is somewhatsimplified. I

Essentially 36, 35 and 34 mirror the first three fonnants of naturalspeech i.e. they resonate at around the natural frequencies of the vocaltract. In the synthesis of an utterance, they are modified in accordancewith the alteration in shape which the human vocal tract would undergoduring speech production.

. 36-34 can be connected in parallel rather than in series ifappropriate modifications are made to the rest of the circuitry of FIG.11.

The term "voice as used in the preceding paragraph means the periodicitysuperimposed on certain speech sounds by the vibration of the vocalchords. The term noise" means the turbulent waveform produced by theflow of air in the constricted vocal tract. Front fricatives" include,for example, the English labiodental fricatives If/ and /v/ and thealveolar fricatives /S/ and /Z/. Back fricatives include the Englishpalatal fricatives /g/ and /g/ and the glottal spirant /h/.

of a single phoneme may be desirable in order to reproduce adequately,for example, the English bilabial nasal /m/, the alveolar nasal In/ andthe velar nasal /ng/ of English sing."

The facility of dissipating energy in the interval between successiveexcitation pulses also has application in the filters 34 (F3),35 (F2)and 36 (F1).

It is to be understood that the foregoing description of specificexamples of this invention is made by way of example only, and is not tobe considered as a limitation on its scope.

lclaim:

l. A speech synthesizer which includes a plurality of resonant circuits,each having inherent damping, each resonating at different frequencies,excitation means associated with each of said resonant circuits forrepeatedly applying momentary excitation pulses to the associatedcircuit thereby causing damped oscillations therein, and means forselectively controlling said excitation means to cause the amplitude ofthe oscillations in any one of the circuits to decay to'an arbitrarilylow value before the application of the next successive excitation pulseto that same circuit.

2. A speech synthesizer as claimed in claim 1, wherein each resonantcircuit has a quality factor, and said excitation control meanscomprises means for lowering the quality factor of each of the resonantcircuits for a portion of the time between the application of successiveexcitation pulses.

3. A speech'synthesizer as claimed in claim 2, including resistancemeans associated with each resonant circuit for damping the associatedresonant circuit, and means for switching a resistance means into theassociated resonant circuit to give critical damping in the resonantcircuit for some discrete time after the application of one excitationpulse, and means'for removing said resistance means from said resonantcircuit before the application of the next succeeding excitation pulse.

4. A speech synthesizer as claimed in claim 1, wherein one of theresonant circuits includes an inductance in parallel with a capacitance.

5. A speech synthesizer as claimed in claim 1, wherein one of theresonant circuits includes an inductance in series with a capacitance.

' 6. A speech synthesizer as claimed in claim 3, further including aninductor and transistor connected in parallel, and means for connectingthe critical damping resistance between said inductance and the emitterof said transistor.

7. A speech synthesizer as claimed in claim 6, wherein the transistor issymmetrical, and wherein there are means for applying a pulse ofpredetermined duration and timing to the base of the transistor toswitch it on. v t

8. A speech synthesizer as claimed in claim 1, and fuither includingmeans for varyingthe bandwidths of the resonant circuits independentlyof one another. 7

9. A speech synthesizer as claimed in claim Landjfurther including aninductance in one of the resonant circuits adjacent one winding of atransformer.

10. A speech synthesizer as claimed in claim 9, further including avariable resistance in series with another winding of the transformer.

11. A speech synthesizer as claimed in'claim 10, wherein the saidvariable resistance is a transistor.

12. A speech synthesizer as claimed in claim 11, wherein the transistoris a symmetrical PNP transistor which is kept saturated and an externalcontrol signal is applied as a potential to the base of the transistor.

13. A speech synthesizer as claimed in claim I, wherein said resonantcircuit has associated therewith a variable. resistance for connectioninto said resonant circuit to presebthe bandwidth of the filter.

